The present invention relates to a method of preparing fine grain multicarbide powders for cemented carbides.
WC-Co-cemented carbides are made by powder metallurgical methods of milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering. The milling operation is an intensive wet milling in mills of different sizes and with the aid of milling bodies which are usually made of cemented carbide. The milling time is of the order of several hours up to days. Milling is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling increases the reactivity of the mixture which further promotes the formation of a dense structure.
Because of the long milling time, the milling bodies are worn and contaminate the milled mixture, which has to be compensated for. The milling bodies can also break during milling and remain in the structure of the sintered bodies. Furthermore, even after an extended milling, a non-homogenous rather than an ideal homogeneous mixture may be obtained. In order to ensure an even distribution of the binder phase in the sintered structure, sintering has to be performed at higher temperature than the theoretical.
An alternative way is to start from an intimate mixture of cobalt and tungsten, which mixture subsequently is carburized. The mixture can be obtained through the formation of a composite metal salt by a chemical process step. U.S. Pat. No. 3,440,035 discloses such a method of preparing cemented carbide powder characterized in that an aqueous solution or suspension of ammoniumparatungstate (APT) and nitric or hydrochloric aqueous solution of, e.g., cobalt, are mixed. The mixture is then subjected to a neutralizing reaction with ammonium hydroxide at a temperature of 20.degree. to 80.degree. C. when the pH-value of the mother solution after the reaction is adjusted and between 4.5 to 8. The resultant fine composite precipitate containing tungsten and cobalt in the desired composition controlled by reaction conditions is filtered, dried by heating and then subjected to reduction and carburization to obtain a WC-Co composite powder in which the WC grain size generally is submicron. An improved method characterized by constant control of the solution pH by continuous addition of ammonium hydroxide or by the use of pH buffers, is disclosed in our Swedish patent application SE 9402548-3 and our concurrently filed co-pending U.S. patent application Ser. No. 08/464,564 (Attorney Docket No. 024444-132).
WC, Co and/or Ni are normally the main components in hard materials. However, other metals from groups IVa, Va or VIa of the periodic system of the elements such as Mo, V, Cr, Ta, Ti and Nb are also added particularly in cemented carbide grades for machining of metals. Ti, Ta, and V are according to the method of U.S. Pat. No. 3,440,035 added as carbides to the composite WC-Co powder after the carburization.